Solution
I got fed up with PIL and switched to Pillow, which is a fork of PIL. It solved the problem for me. Remove PIL and install Pillow:

sudo pip uninstall PIL
sudo pip install pillow -U

Pillow produces the following statistics:

--------------------------------------------------------------------
SETUP SUMMARY (Pillow 2.0.0 fork, originally based on PIL 1.1.7)
--------------------------------------------------------------------
version 2.0.0 (Pillow)
platform linux2 2.7.4 (default, Apr 19 2013, 18:28:01)
[GCC 4.7.3]
--------------------------------------------------------------------
*** TKINTER support not available
(Tcl/Tk 8.5 libraries needed)
--- JPEG support available
--- ZLIB (PNG/ZIP) support available
*** TIFF G3/G4 (experimental) support not available
--- FREETYPE2 support available
*** LITTLECMS support not available
*** WEBP support not available
--------------------------------------------------------------------

Problem
With Splinter you can automate a browser window (click on a button, type in some text, etc). You can also use a Firefox instance beside Chrome and some other browsers. But how to open the Firefox instance in fullscreen (as if you had clicked on the “maximize” button)? Strangely, there is no command-line option for this :(

Solution
Well, under Linux there are some tools that allows you to interact with windows:

xwininfo

xdotool

wmctrl

When the Firefox instance is opened, it becomes the active window and I ask its window ID with “xdotool getactivewindow”. Then, with “wmctrl” I can toggle this window to fullscreen.